Influence of thaxtomins in different combinations and concentrations on growth of micropropagated potato shoot cultures

Plant-pathogenic Streptomyces species produce a variety of different phytotoxic 4-nitroindol-3-yl-containing 2,5-dioxopiperazines (thaxtomins) that induce scab symptoms on potato tubers (Solanum tuberosum). The possible mutual synergistic or antagonistic effects of thaxtomins are unknown. Modified methodology using column chromatography allowed the purification of thaxtomin A in large quantities (27 mg, HPLC purity of 97%). Thaxtomin A ortho isomer, thaxtomin B, and C-14 deoxythaxtomin B (thaxtomin D) were also purified. All four compounds induced similar symptoms of reduced root and shoot growth, root swelling (10-200 ppb), or necrosis (200-1000 ppb) on micropropagated in vitro cultures of potato. The scab-resistant potato cvs. Sabina and Nicola were more tolerant to thaxtomins than was the scab-susceptible cv. Matilda. Thaxtomins applied in combinations showed additive effects but no synergism, whereas thaxtomins A and B displayed antagonism with thaxtomin A ortho isomer.